Physics of Laser Cutting

 

Laser Machining Processes

Several major laser machining processes are illustrated below. (The notable omission is welding, beyond the scope of this document.)

 

drill_groove_cut.gif (44276 bytes)

(Chryssolouris, 1991)

 

This diagram illustrates the cutting process in cross section. Note that a gas assist can be used to speed cutting of metals.

laser_through_cutting.gif (27067 bytes)

(Chryssolouris, 1991)

 

Drilling Rates

A simple method of analysis is proposed in (Wilson, 1987).

The energy to vaporize a mass m of solid material at intial temperature T is

Ev = m(CS(Tm-T)+CL(Tv-Tm)+Lf+Lv)

CS = solid specific heat capacity

CL = liquid specific heat capacity
Tm = melting point
Tv = boiling point
Lf = latent heat of fusion
Lv = latent heat of vaporization

Usually, Lf << Lv and T << Tv , and Cs CL = C. We then have the simple form Ev = m(CTv+Lv). Now, consider a circular laser beam of area A boring into the surface of such a material with a velocity vs directed into the material. It must remove a section of mass vsr A per unit time. Ignoring reflectance from the material surface, the heat flow is equal to the beam power P. Assuming a beam with diameter d and equal power over A, we have P = vsrho.gif (873 bytes)(p d2/4)(CTv+Lv).

If vs exceeds the normal rate of heat diffusion into the material, this equation is fairly accurate for estimating drilling rates or hole depths. To find hole depths, solve for the quantity vst, where t is the duration of the beam pulse. For example, consider a 100-msec pulse from a 10W laser with a beam diameter of 1mm. If this were to strike a Perspex (methyl methacrylate) sheet, the resultant hole would have a depth of 1.6mm.

Note the inverse-square dependence of hole depth on beam diameter. Halving the beam diameter results in a hole four times deeper. This highlights the importance of beam focusing in laser machine design.

 

Cutting Rates

This model can be used to estimate cutting rates as well. Consider the laser scanning over the surface of the material with velocity vb. As it scans, it cuts through the material to a depth z = vsd/vb. We now have

P = (p /4)zvbr rho.gif (873 bytes)d(CTv+Lv)

The following table can then be used to approximate the laser power necessary to cut a given material.

wpe1.jpg (18704 bytes)

(Wilson, 1987)

 

(Chryssolouris, 1991) describes a model that accounts for the material absorptivity as well.

s = cutting depth
a = material absorptivity
P = laser beam power
rho.gif (873 bytes) = material density
v = scanning velocity
d = beam spot diameter
cp = specific heat
Tv = temperature at surface (melting temp.)
T = temperature of ambient
L = latent heat of fusion

Note the following:

  • The cutting depth is proportional to P/vd, which is the energy input per area of workpiece.
  • Cutting depth is small for materials with a high melting point and a high latent heat of evaporization.